This invention relates to the use and formation of composite films through electrodeposition and anodization techniques. More specifically, the invention relates to the electrochemical formation of polymer-metal oxide composite films utilizing an electrolyte which incorporates a conductive polymer.
A common anodizing process employs aluminum as a substrate. The aluminum anodizing process is most often used to produce decorative finishes, to increase the corrosion or wear resistance of the aluminum substrate, or to provide an adherent interface for subsequent coatings. In most cases, the anodic film requires supplementary processing after film formation to achieve these characteristics. Supplementary coating is carried out through various sealing processes and conversion coatings, which seal the porous structure of the as-anodized film to offer corrosion resistance, pigmentation, and/or to provide lubricity to enhance wear resistance.
When the anodic film is used as an adherent interface for subsequent coatings, its purpose is usually to join dissimilar metals. There has long been a need for a reliable means to chemically join dissimilar materials whose atomic structures and compositions render them chemically immiscible, such as metals, ceramics and polymers.
Coatings used to enable a ceramic-to-metal joinder typically possess constituents which are miscible with their deposant substrates. For ceramic-to-metal joining, these constituents are metal oxides and glass formers which wet and bond to the ceramic surface. These coatings also include additional immiscible constituents which, by virtue of their immiscibility, create a new surface on which the joining process can be performed. Known methods to provide these coatings, such as thick and thin film metallization techniques, form a composite interface between the faying surfaces which permits complete chemical bonding of dissimilar metals and materials. However, these methods have not permitted polymer-to-metal joinder employing a chemical bond.
Some of the most common polymer-metal bonds use adhesives. These bonds require neither miscibility nor the formation of intermediate phases. The strength of the resulting polymer-metal bond employing an adhesive normally hinges on the quality of the substrate surface preparation. This is because the adhesive, while uncured, will flow to fill the features of the surface morphology. In this fashion, a mechanical bond between the adhesive and the substrate surface has been formed. While some of the bond strength is derived from polar forces between the adhesive and the surface, these forces are relatively minor and do not contribute in any meaningful fashion to the overall integrity of the bond.
"Adhesiveless" polymer--metal bonds have also been developed in the electronics industry. These bonds provide the advantage of size reduction, as well as enabling increased flexibility of electrical connectors and circuits. Adhesiveless bonds may be achieved by "seeding" a chemically prepared polymer surface. The nature of the adhesiveless bond involves the binding of a noble metal salt to a functional ligand on the polymer surface, followed by reduction of the noble metal to a zero valence state. The surface becomes slightly conductive, which enables electroless metal deposition. The resulting metal surface can then be coated by way of electrodeposition. However, the seeded film is insufficiently conductive for direct use for electroplating. Thus, without the enhanced surface preparation necessary to enable electrodeposition, the adhesiveless bond forces are weak and peel strengths are low.
The typical failure mode for both adhesive and adhesiveless polymer-metal bonds is delamination or "peeling" of the adhesive or one of the faying surfaces from the mating interface. Failures occur due to insufficient or inadequate surface preparation, surface contamination, or the use of a misapplied, worn, outdated or otherwise deficient adhesive.
Surface preparation for polymer--metal bonding ranges from simple surface cleaning to the development of a supplementary conversion coating on the metal surface. For steel bases, phosphate-type conversion coatings are most commonly utilized. For aluminum bases, the surface is often anodized. If properly deposited, the nature of the conversion coating or anodic film is that of a metal phosphate layer or a metal oxide layer chemically bound to the metal substrate. However, such coatings act only as a surface enhancer to promote adhesion for the polymer attachment. In other words, the conversion coating/anodic film acts as a primer and, while chemically bound to the metal substrate, it is not chemically bound to the subsequent polymer coating.
Anodic coatings used as "stand-alone" films, deposited for corrosion and wear resistance or for decorative purposes, but not to provide a dissimilar material joinder, have been created using a two-step process in which a polymer or other material is applied to the anodic film surface after anodizing has occurred. With polymer-based supplementary coatings, the polymer is not chemically bound to the oxide film and is of a thickness limited by the following factors: the effective mechanical adhesion properties of the film to the oxide; the diameter of the pores in the oxide film; surface wetting characteristics of the oxide; and the viscosity of the polymer coating. Because the supplementary coating is of a finite thickness that does not fully intrude the porous structure, it can chip and wear away from the substrate surface during service and, therefore, has a limited useful life. In another process, known as the "Metalast" process and disclosed in U.S. Pat. No. 5,132,003 to Mitani, an acrylate polymer is electropolymerized following hard coat anodizing. However, in this process, the acrylate polymer does not actively participate in the anodizing reaction, and requires a subsequent treatment from a second electrolyte bath which incorporates a metal salt, forming a finished composite coating in three steps. Other supplementary coatings, placed to impart corrosion resistance, involve conversion of the oxide into a metal complex, the most common being chromate conversion coating. As deposited, these coatings are gelatinous and therefore fragile. With dehydration, the supplementary coating becomes more durable but the useful life of the coating is limited by the coating thickness and by the amount of abrasion the component experiences during service.
In two publications, Huang, W. S. et. al., Polyaniline, A Novel Conducting Polymer--Morphology and Chemistry of its Oxidation and Reduction in Aqueous Electrolytes, Journal of the Chemical Society, Faraday Transactions I, 92: 2385-2400 (1986), and Chiang J. C. et. al., `Polyaniline`: Protonic Acid Doping of the Emeraldine Form to the Metallic Regime, Synthetic Metals, 13: 193-205 (1986), it is described how polyaniline can be transformed from the insulative to the conductive regime by doping the polymer with protonic acids. In this fashion, an already-polymerized film of polyaniline can be electrochemically or chemically doped to yield a conductive surface for subsequent processing. The reaction is reversible; therefore, by changing the external exposure parameters, one can dope to make the polyaniline conductive and "de-dope" to make it insulating. The doping processes involve an oxidative polymerization reaction where the protonic acid is bound to the polymer backbone through ring sulfonation, "de-doping" is a reduction reaction, as shown in FIG. 1.
The use of electropolymerized polyaniline as a surface conductive layer has been studied. Electropolymerization has been shown to occur on already-formed polyaniline films as well as in an electrodeposition reaction from electrolytes which contain aniline monomers in solution with protonic acids.
V. P. Parkhutik et. al., "Deposition of Polyaniline Films onto Porous Silicon Layers", Journal of the Electrochemical Society, Vol. 140, No. 6 (June, 1993), describe a process by which thin layers of conductive polyaniline are electrodeposited from sulfuric acid solutions onto already anodized porous silicon layers, developed at 2.0 A/dm.sup.2 with pore diameters of about 4 nm. This publication indicates that the films developed on the anodized silicon cathodes exhibited good adhesion, acid resistance and infrared structures typical for the conductive emeraldine oxidation state of polyaniline. A polymerization potential of +0.6 to +1.0 v. SCE is also described. However, no actual silicon-polyaniline bond is documented. Also, the Parkhutik et. al. study, as well as U.S. Pat. No. 4,943,892 to Tsuchiya, for example, disclose a 2-step (anodization, followed by electropolymerization) process. In these references, electropolymerization is carried out by dipping the already anodized workpiece into a solution containing the appropriate concentration of protonic acid and aniline monomer and initiating the polymerization reaction at the workpiece surface by applying the characteristic voltage for the desired oxidation state of polyaniline, or by cycling the workpiece, as prepared for electropolymerization, through a series of voltages characteristic for the various phases of polyaniline. In these studies, the resultant polymer film, as deposited, exhibited the characteristics of the conductive emeraldine phase of polyaniline. Additional United States patents which describe this or a similar process with various applications are: U.S. Pat. No. 4,769,115 (Masaharu); U.S. Pat. No. 5,422,194 (Masaharu); U.S. Pat. No. 5,556,518 (Kinlen); and U.S. Pat. No. 5,567,209 (Kobayashi).
Researchers would have been dissuaded by the use of an aluminum-polyaniline reaction to form an anodized coating because the standard aluminum anodizing potentials exceed the published polymerization potentials for polyaniline. This raises the concern that the polyaniline molecule will degrade during anodization. Degradation is thought to occur by way of cleaving the carbon-nitrogen or carbon-hydrogen bonds of the monomer within the electrolyte during anodizing. More specifically, there is a concern that polyaniline can degrade to hydroquinone at potentials above 0.8 volts and, therefore, might have no impact or meaningful interaction with the anodic film.
Thus, electropolymerization and utilization of the polymer film as a surface conductive layer has been studied. Other publications describe utilizing the conductive layer as a precursor for subsequent metal electrodeposition. See, e.g., Angelopoulos, et. al., Conducting Polyanilines: Applications in Computer Manufacturing, Proceedings of the SPE 49.sup.th Annual Technical Conference & Exhibits, 765-769 (1991), incorporated by reference. However, none describe the formation of a composite metal oxide-polymer film through anodization of the metal with the polymer deposited simultaneously from a polymer solution within the electrolyte.
It would, therefore, be advantageous to provide an anodized coating which essentially eliminates the use of an adhesive attachment for subsequent polymer coatings. It would also be desirable to provide a self-sealing, stand-alone, chemically-bound polymer-to-metal coating in a single step, which would yield substantial time and material savings while providing an industrially viable process. Particular utility would also be found in the use of a stand-alone polymer-metal oxide composite coating chemically bound to a metallic substrate achieved through a standard anodization process; since the polymer phase would be completely and homogeneously integrated within the metal oxide, such a coating would provide superior wear and corrosion resistance.